The present invention is directed to the strain gage arts. It finds particular application to a generally flexible strain gage having a semiconducting stain sensing element and will be described with particular reference thereto.
Strain gages are often used to sense strain in a subject material. The strain gage has a strain sensing element that is attached or adhered to the subject material. When the subject material is strained, the resistance of the sensing element changes in proportion to the strain it experiences. The change in resistance in the sensing element as it is compressed or elongated is measured and used to calculate strain in the subject material. Foil strain gages, which have a metal sensing element, are often used to measure strain. However, metal sensing elements have a relatively low gage factor, which reduces the sensitivity of the gage. The use of a semiconductor material, such as doped single crystal silicon or polycrystalline silicon, as the sensing element increases the gage factor of the strain gage dramatically. However, because most semiconductor materials are generally fragile, a semiconducting sensing element is prone to fracture. In order to protect the semiconducting strain sensing element, it is typically mounted upon a rigid backing to provide support (termed a xe2x80x9cbackedxe2x80x9d gage). This rigid backing prohibits use of the gage on curved surfaces. Alternately, an unbacked gage may be used wherein the sensing element is directly adhered to the material. However, unbacked gages are difficult to mount, and the sensing element is exposed and thus still prone to fracture. The unbacked gages are too fragile to mount on curved surfaces. Accordingly, there is a need for a strain gage having a semiconducting strain sensing element, wherein the strain gage has a generally flexible substrate and/or sensing element to provide ease of handling and enable the gage to be used on curved or irregular surfaces.
Many difficulties arise when trying to form a single crystal semiconducting material or polycrystalline material on a flexible substrate, such as polyimide. It is known that amorphous silicon may be adhered to a flexible substrate using glow-discharge decomposition, but this process can not be used with other forms of silicon. Instead, single crystal or polycrystalline silicon must be deposited or grown by different methods, such as epitaxial growth. However, epitaxial growth requires temperatures of around 950xc2x0 C., and a polyimide substrate decomposes around 550-580xc2x0 C. Polycrystalline silicon deposition also occurs at temperatures above 500xc2x0 C. Thus, epitaxial growth is not feasible for use with polyimide substrates. Furthermore, epitaxial growth can only deposit silicon on an already-existing layer of single crystal silicon, and therefore this process cannot be used to deposit single crystal or polycrystalline silicon on a flexible substrate.
The present invention is also directed to a method of manufacturing a sensor, and more particularly, to a method of manufacturing a generally flexible strain gage. When manufacturing a backed sensor, generally the sensor or sensing element is oriented on the substrate or backing, and the sensor is then fixed to the substrate. This process requires high precision instruments or a trained individual to locate the sensing element in the desired location and orientation. The sensing element is quite difficult to handle due to its brittleness and small size. Accordingly, there is a need for a method of forming a sensor which minimizes or avoids having to handle, locate or attach the sensing element.
The present invention is a generally flexible strain gage incorporating a relatively thin semiconducting sensing element mounted to a generally flexible substrate. The strain gage of the present invention has a flexible backing, which makes it easier to mount and enables the gage to be conformed to curved surfaces. The strain sensing element is thin enough to be flexible, and is made from a semiconducting material, such as doped single crystal silicon, which provides a high gage factor relative metal foil, or amorphous silicon as in Uchida U.S. Pat. No.; 4,658,233. In a preferred embodiment, the invention is a generally flexible strain gage comprising a semiconducting single crystal strain sensing element or a semiconducting polycrystalline strain sensing element, and a generally flexible substrate supporting the strain sensing element.
The present invention is also directed to a method for manufacturing a sensor or sensing element mounted on a flexible substrate. In the present method, the sensing element is formed on a wafer, and the flexible substrate is then formed about the sensing element. Once the flexible substrate is cured, the wafer is etched to a precise depth to remove the bulk of the silicon substrate and expose the sensing element. The etch is preferably performed using dry etching techniques, since wet etching can damage sensing elements on the unetched side of the wafer. Furthermore, Reactive Ion Etching (RIE), and preferably Deep Reactive Ion Etching (DRIE) is the dry etch process of choice because it offers high etch rates and high selectivity to etch stop materials.
Because the sensing element is formed directly on the wafer, the sensing element is anchored in the desired location. The substrate can then be formed about the sensing element. In this manner, the sensing element also need not be directly handled or located. In a preferred embodiment, the invention is a method for forming a generally flexible strain gage comprising the step of selecting a wafer having a portion of a base material and portion of a single crystal semiconducting material or polycrystalline semiconducting material located thereon. The method further comprises the steps of etching a strain sensing element out of the semiconducting material and forming a generally flexible substrate onto said sensing element.
Other features and advantages of the present invention will be apparent from the following description, with reference to the accompanying drawings and claims, which form a part of the specification.